(well-known Jupiter moon)
Ganymede (Jupiter III) is Jupiter's largest moon
and the third outward from Jupiter of the original four moons
found centuries ago, but the seventh according to current
knowledge. Its radius is 2634 km (.4 Earth's), its mass is
1.5×1023 kg (0.025 Earth's), with a mean density
about twice that of water (about 1/3 Earth's)
and subsurface oceans are suspected.
Ganymede's orbital period is 7 days and it is tidally locked.
Its orbit is in sync with Europa (2x) and Io (4x).
Flyby observations of Ganymede have been made by
the Pioneer, Voyager, and Galileo missions, the latter
within 264km of its surface.
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